byte enables are used to send only the relevant information.
lets assume that we have a 1byte data width on the other side of the bridge, and a 2byte data width on the axi side of the bridge, with the bridge using a single clock.
Now, the bridge should be informed about which byte of information should go to the other side,since the other side can only support one byte of data. for that purpose, we use byte enable.
alternative would be to use a fifo and a read clock twice as fast as the axi write clock, but with no phase dif ference(hope i'm right).